A volatile memory device, such as a dynamic random access memory (DRAM) device, performs refresh operations for maintaining stored data. A refresh period may be defined by SPEC (specification). When a DRAM memory cell has a data retention time that is shorter than the refresh period defined by SPEC (e.g., a weak cell), the weak cell may be with a redundancy cell. Thus, a refresh method capable of reducing the number of redundancy cells and improving a repair ratio by the refresh leveraging is needed.